1. Field of the Invention
The present invention relates to the extraction and separation of uranium, thorium and rare earth values, and, more especially, to the extraction/separation of such values by certain treatment of aqueous solutions of the chlorides thereof.
2. Description of the Prior Art
It is known that uranium, thorium and rare earths are present in the natural state in ores such as monazite, bastnaesite and xenotime. Such values are also found in various extraction solutions, which are subjected to prior concentration. In order to recover some of these elements, the ores are subjected, after grinding, to alkaline attack by sodium hydroxide in an autoclave. The uranium, thorium and rare earth hydroxides formed precipitate, while the soluble phosphates are removed.
The hydroxides formed also include various metal hydroxides deriving from the metals contained in the starting material ores, such as iron, titanium and calcium.
The uranium, thorium and rare earth hydroxides and, if appropriate, the various other metal hydroxides are solubilized by means of a strong acid, such as nitric acid or hydrochloric acid.
One technique for separating the uranium, thorium and the various rare earth values consists of carrying out successive liquid-liquid extraction operations beginning with aqueous solutions of the salts of said elements.
The uranium, the thorium and also the various rare earth values which have been separated cannot, however, be used in certain applications unless they contain only a very small level of impurities and of radioactivity.
It too is known to isolate uranium, thorium and rare earths from niric acid solutions [Menzies and Rigby, J. Appln. Chem., "Solvent Extraction of Thorium" (Mar. 11, 1961)].
And it is also known, from Ekstraktsiya-Theoriya, Primenemie, Apparatura, Sb. Statei, No. 2, 160-4 (1962), to extract thorium from hydrochloric acid solutions by means of an organic extractant consisting of diisoamyl methylphosphonate diluted in an organic solvent.
The authors of the immediately aforesaid article indicate that the extraction coefficient of the thorium increases with the hydrochloric acid concentration and decreases with increasing thorium concentration.
This conclusion is confirmed in an article published in the journal Fresenius'Z. Anal. Chem., 238, 4 (1968), in which the authors extract and separate thorium and cerium from hydrochloric acid solutions by means of a neutral organophosphorus extractant consisting of tributyl phosphate or diisoamyl methylphosphonate in solution in an organic diluent, namely, xylene. The authors studied the variation in the respective extraction coefficients of the cerium and thorium as a function of the variation in the hydrochloric acid concentration. They conclude from this study that, to obtain a proper separation of cerium from thorium, the hydrochloric acid concentration in the aqueous starting solution must be at least 6 N.
It is thus apparent that, according to these two articles, the extraction and separation of the two elements thorium and cerium from hydrochloric acid solutions can only be carried out as a practical matter in the case where the aqueous solutions have a high acidity and a low concentration of the elements to be extracted.
Now, the solutions originating from the attack of the ores by sodium hydroxide, followed by the dissolution of the oxides with hydrochloric acid, are neutral or very weakly acidic solutions (hydrochloric acid concentrations generally less than about 2 N) possessing a high concentration of the elements to be separated (uranium, thorium and rare earths).
The extraction of neutral or very weakly acidic solutions possessing a high concentration of the elements to be separated (uranium, thorium and rare earths), with a view to obtaining each of these elements in the purest possible state at the lowest cost, is an industrial problem. In fact, uranium, thorium or rare earths cannot be used in certain applications unless they have a very low level of impurities.